Chromosomes of Bromus auleticus Trin. ex Nees (Poaceae)

SHORT COMMUNICATION

Chromosomes of Bromus auleticus Trin. ex Nees (Poaceae)

Gilmar Efrem Martinello^I; Maria Teresa Schifino-Wittmann^II

^IUniversidade Estadual do Rio Grande do Sul, Cachoeira do Sul, RS, Brazil

^IIUniversidade Federal do Rio Grande do Sul, Faculdade de Agronomia, Departamento de Plantas Forrageiras e Agrometeorologia, Porto Alegre, RS, Brazil

^{Correspondence}

ABSTRACT

The chromosome number of 14 accessions of Bromus auleticus Trin. ex Nees, native of Rio Grande do Sul, was 2n = 6x = 42, same ploidy level found in other South-American Bromus species. Its chromosomes were metacentric or submetacentric, ranging from ca. 4 mm to ca. 8 mm in length. Up to two satellite-bearing chromosome pairs were sometimes observed. However, as already reported for other species, the high symmetry and homogeneity of the karyotypes made it difficult to detect possible intraspecific differences.

Key words: Bromus auleticus, chromosome numbers, native forages.

Introduction

The genus Bromus comprises more than 100 annual and perennial species (Mabberley, 1997) distributed around the world, typically cool-season grasses, varying greatly in adaptation and use, and including some important forage and range species, such as the perennials B. inermis, B. anomalus, B. pumpellianus, B. catharticus, and the annuals B. mollis and B. rigidus, among others (Carlson and Newell, 1985).

Chromosome numbers vary from 2n = 14 to 2n = 84, most of the species being diploid (2n = 14) or tetraploid (2n = 28) (Federov, 1969). Chromosome size may be very different among species (Armstrong, 1983; Schifino and Winge, 1983). Ploidy levels may vary within the same species, as in B. diandrus and B. sterilis (Oja and Laarmann, 2002). The chromosome complements consist mainly of chromosomes resembling each other in size and centromere position (Joachimiak et al., 2001). C-banding techniques (Joachimiak, et al., 2001; Tuna, 2001) have shown that almost all the species analyzed present similar amounts of heterochromatin, mainly telomeric, albeit some small amounts of intercalary heterochromatin were detected. Therefore, distinction between different karyotypes is generally made based on the size of satellites (Armstrong, 1983; Joachimiak et al., 2001).

In Rio Grande do Sul, Southern Brazil, the genus is represented by the native B. auleticus, B. brachyanthera, B. catharticus, and the exotic B. commutatus and B. mollis species (Longhi, 1977). Among these, the perennial allogamous B. auleticus, popularly known as "cevadilha" or "cevadilha vacariana" (Araújo, 1971; Mohrdieck, 1973), occupies an outstanding position as a forage in the native pastures of that State. This species has called the attention of plant breeders and agronomists (Mohrdieck, 1993; Soares, 1999, among others), and some populations have already been characterized by isozyme and RAPD analysis (Yanaka, 2002). Cytological data on this species, however, is scarce. Only isolated chromosome counts in a few plants (2n = 42) have been performed (Elliott, 1948 and 1949, in Federov, 1969; Bowden and Senn, 1962) and, to our knowledge, no other cytogenetic information on the species has been published.

This paper reports data on chromosome numbers and morphology of 14 natural populations of B. auleticus from the State of Rio Grande do Sul.

Material and Methods

Seeds were collected at several locations of Rio Grande do Sul, Southern Brazil (ca. 33° to 28° S) (Table 1). Each sample from each collection site was considered as an accession. Seeds from 11 of these accessions were kindly provided by the germplasm bank of EMBRAPA-CPPSul (Empresa Brasileira de Pesquisa Agropecuária - Centro de Pesquisa de Pecuária dos Campos Sul-Brasileiros).

For somatic chromosome counts, seeds were germinated in Petri dishes with moistened filter paper, and the 1- 1.5 mm long roots were pre-treated with a saturated solution of paradichlorobenzene (PDB) for 18-20 h at 4 °C, fixed in 3:1 ethanol-acetic acid for 24 h, and stored in 70% ethanol below 0 °C until required. For slide preparation, root tips were hydrolyzed with HCl 1 N for 10 to 15 min at 60 °C, stained with Feulgen for ca. 2 h, and squashed in a drop of acetic carmine. Semi-permanent slides were examined by light microscopy. Only intact and well-spread metaphases were analyzed. Each root tip was considered as one individual.

Results and Discussion

All the 220 cells of the 90 analyzed individuals of the 14 accessions presented 2n = 42 chromosomes (Table 1, Figure 1). The ploidy level of B. auleticus (hexaploid) is the same as in other native South-American species, such as B. bonariensis, B. brevis, B. parodii, B. brachyanthera var. uruguayensis, B. catharticus (Schifino and Winge, 1983; Naranjo, 1985).

Chromosome size ranged from ca. 4mm to ca. 8mm, and they were all either metacentric or submetacentric. Detailed karyotypic analyses were not performed, but chromosome morphology was very similar to that reported for several Eurasian and North-American Bromus species (Schulz-Schaeffer and Markarian, 1957; Armstrong, 1983; Joachimiak et al., 2001; Tuna et al., 2001), as well as for the South-American species B. catharticus (Schifino and Winge, 1983; Naranjo, 1985), B. brevis, B. parodii, and B. bonariensis (Naranjo, 1985).

Up to two satellite-bearing chromosome pairs could be seen in a few cells of some accessions, but in most cells the satellites were very difficult to distinguish. Armstrong (1982) already pointed out the difficulty in detecting satellites when they are small or when the chromosomes are very condensed. Naranjo (1985), reviewing data from the literature, commented that there might be actual differences in the number of satellite-bearing chromosomes (or in the number of these chromosomes expressed) among different Bromus individuals or species, but that the high symmetry and homogeneity of the karyotypes could also make it difficult to detect possible differences.

A pilot FISH (fluorescent in situ hybridization) test performed in one B. auleticus accession (data not shown) suggested the presence of up to fourteen 45 S rDNA sites.

Bromus chromosomes are a good material for cytogenetic analysis, regarding for example their size, however the little success obtained so far in detecting variability with conventional staining and C-banding techniques suggests that more advanced techniques should be employed.

Acknowledgments

The authors thank Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq) for the post-doctoral fellowship granted to the G.E.M., and Dr. Klecius Ellera Gomes and MSc José Carlos de Oliveira from EMBRAPA-CPPSul for supplying the seeds.

Editor: Yatiyo Yonenaga-Yassuda

Accepted: May 29, 2003

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Publication Dates

• Publication in this collection
  29 Sept 2003
• Date of issue
  2003

History

• Accepted
  29 May 2003
• Received
  26 Feb 2003